Viruses are one of the major stress sources for plants. It is not seldom that crop-producing districts shift or cultivars are renewed due to damage from viral diseases. At present, there is no effective drug that acts on viruses directly. Thus, plants infected with viruses are subjected to incineration. Although virus resistance is an important goal for breeding, it has been impossible to rear a virus resistant cultivar with conventional breeding technologies such as crossing where a source of a resistance gene cannot be found in wild-type or related species.
As a method for supplementing such conventional breeding technologies, a method have been developed recently in which virus resistance is conferred on plants using recombinant DNA technology. Recombinant DNA technology has made gene transfer possible which goes beyond the deadlock of conventional crossing described above. Furthermore, this technology allows to introduce into existing cultivars a virus resistance gene alone and, thus, it has become possible to save the time required for breeding greatly.
As a method for creating virus resistant plants using recombinant DNA technology, methods of expressing a gene encoding a virus coat protein or a viral replication protein, an antisense gene, and a gene encoding a satellite RNA, and the like have been reported (see, for example, Arch. Virol., 115, 1, 1990). These methods confer resistance to only one kind of virus or its related viruses alone. A method for conferring resistance to various kinds of viruses is still under development.
As a method for conferring resistance to a large number of viruses at the same time, a method using a double-stranded RNA specific ribonuclease (International Publication WO93/20686) and a method using a (2'-5')oligoadenylate synthetase (Bio/Technology, 11, 1048, 1993) have been reported.
With respect to the method using a (2'-5')oligoadenylate synthetase, it has been reported that the virus resistance in the resultant transformant plants is extremely weak (The Proceedings of XVIIIth Eucarpia Symposium, Section Ornamentals, 1995). Briefly, a (2'-5')oligoadenylate synthetase was introduced into a tobacco plant (cv Samsun) and then the tobacco mosaic virus (hereinafter referred to as "TMV") OM strain was inoculated into the resultant plant expressing the (2'-5')oligoadenylate synthetase. As a result, no delay in the development of disease symptoms was recognized as compared to controls. Also, there have been many reports that the existence of ribonuclease L-like molecules are not recognized in plant cells (Biochem. Biophys. Res. Commun., 108, 1243, 1982; J. Biol. Chem., 259, 3482, 1984; The Proceedings of XVIIIth Eucarpia Symposium, Section Ornamentals, 1995).